The Observer Effect

Dr.Todd

I keep seeing references to the observer effect in the double -slit experiment where the a
of observing the photon or electron going through the slits causes a collapse of the wave function. so, instead of getting a cool interference pattern, you get the pattern expected if the light was acting as a particle.

Has this actually been tested experimentally?

If so, I have not been able to find it anywhere.

If anyone can post a citation from a peer-reviewed physics journal where this phenomenon is reported, I would appreciate it.

thanks,

Dr. Todd

olgranpappy

How about (among many): Am. J. Phys. 57 117 (1989)

LoveLab

olgranpappy,
My understanding of the article you cite is that Tonomura et al demonstrated the interference pattern resulting from single-electrons being directed one at a time through the double slits....that experiment did NOT involve showing how the pattern reverts to two straight lines when the electrons are observed going through the slits.

jtbell

Actually, you get (or are supposed to get) the single-slit diffraction pattern, which still depends on the wavelike behavior of light. More precisely, you get two such patterns, one from each slit, superimposed on each other. But the spacing between the slits is normally much smaller than the "width" of the single-slit pattern from each slit, so the two patterns almost coincide.

(I don't have any references to experimental evidence at hand. I just wanted to clear up what would be expected.)

LoveLab

Okay, so you're saying that due to the observer effect, and the resulting collapse of the wave function, that instead of an interference pattern there results a single-slit diffraction pattern (or more precisely, two that are nearly superimposed).

I don't doubt that the hypothetical difference between the double-slit interference pattern, and the superimposed single-slit diffraction pattern can be mathematically determined, with utmost precision.

But diffraction is basically interference, and just to look at them, the double-slit and single-slit patterns are pretty similar, no?

The reason I'm saying this, is because the result of any double-slit experiment that has actually been done, and that I can find reference to, is visually EXTREMELY rough...well, and then to add the extra experimental burden of illustrating the observer effect (which these experiments were NOT intending to show)...well, I don't think you'd really be able to see any difference.

In order for this ever NOT to be simply a thought experiment, the imaging technology would have to be vastly improved. But hey, the Tonomura experiment was done almost 20 years ago, surely we've got better capabilities??

olgranpappy

two straight lines? Not sure what you're saying, but why don't you just go and Google "double slilt experiment" for yourself. you'll find plenty of stuff.

olgranpappy

... or better yet, sign up for a lab course and do the experiment yourself.

LoveLab

Well olgranpappy, I'd love to conduct that experiment myself... But, not possible for me at this juncture. Time for me to freely admit, I'm not a university physics student.

It still begs Dr. Todd's original question....this experiment has NOT been done?? I've googled this extensively, and especially have read in detail about the experiment you cited...

I'm referring to what Dr. Todd mentioned:which is what's supposed to happen when the attempt is made to observe the particles going through the slits, because the wave function is collapsed due to this observation.

Feynman described the way a detector could be placed at each slit, and electrons detected passing through one or the other, and that simply by performing this detection of which slit the electron passes through, the interference pattern disappears, and the pattern is instead what would be expected from particle behavior. This was often presented by Feynman as a **thought** experiment, to describe in a nutshell fundamental quantum paradoxes.

So, in the experiment conducted by Tonomura at Hitachi Labs [Am. J. Phys. 57 117 (1989)] they simply demonstrated the wave-behavior of electrons fired one at a time through double slits. They did not additionally demonstrate the disappearance of the interference pattern when an attempt is made to determine which slit each electron passes through....i.e., the "observer effect".

confusedashell

check the afshar experiment he showed "observer effect" and "many worlds" is misinterpretations (*watching out for haters*) :P

K.J.Healey

Remember though that you can't just place some "detectors" at the slits without COMPLETELY changing the experiment. You can't measure something without changing it.

RandallB

QUOTE=K.J.Healey] Remember though that you can't just place some "detectors" at the slits without COMPLETELY changing the experiment. You can't measure something without changing it. [/QUOTE] Of course you can, that is the whole point of the observer effect (Which is NOT what Afshar was doing in his Which Way effort).
The problem was found early in the 20th century using electrons, and was fundamental to the whole “Which Way” Issue.

Using electrons sent one at a time through a double slit they detected dispersion with interference. The idea was to detect (even if it molested) any electrons going through slit B with a beam of photons that trip a detector just like the seeing eye on your garage door opener.
Sure that would change those electrons so the checking the screen hits when they got B side detections they were not interested in the results of those “molested” electrons. They want to eliminate those results and only consider the remaining Screen hits, of the same number as the B side hits but Known not to go through the B side. By eliminating all B side hits they then knew they only had A side electrons. Even rate of production of these A side unmolested electrons was confirmed the same as the B side screen hits with detection.
Now they had a collection of A side electrons going through while the B side slit was still open, yet unmolested and unchanged.
The result – same dispersion pattern, but without the hoped for interference pattern imbedded within it. By obseving nothing, they affected the results.

This pre-slit detection was the first to display the Which Way problem. Even bad documentaries like “What the Bleep” can describe this.
All the OP is looking for is a reference to a real experiment of someone producing these results -- who when and where did they do it.
Doesn’t someone out there know of a paper or text book that credited someone as actually doing the in front of slit detection experiment? Whitout it how could have QM even known of the paradox needing to be solved.

AhmedEzz

you said "bad documentaries like what the bleep"...do you know any better ones?

back on topic: I think that the observer effect is in the act of measurement itself since measurement cancels the superposition of probability amplitudes and results in determination, that's why we don't get interference with the observer effect in action. Thus it doesn't depend on how it was measured but rather on the measuring itself.

However, i too don't know of any actual experiment or paper that says so.

RandallB

Try any Particle Physics or Astrophysics program on PBS, Discovery or The History Channel, - String Theory included.

As to “back to the topic” - the topic was not questioning the philosophies or various interpretations of what the observer effect might mean. Or even documentation of “entangled” delayed choice types of observer effect observations.
The OP was very specific; where is any documentation of real experiments using observations of elections as they go though a double slit on the way to a dispersion or interference pattern being observed on a detection screen.
Preferably, who was among the first to actually correlate screen hits with the failure to detect them going though with other electrons using one slit, therefore confirming they had to have gone though the other slit unobserved. Who where when and how was this first experimental test of knowing which electrons went through one of two slits without directly observing them, and that gaining this knowledge was enough to destroy the ability to find the inference pattern for those electrons.

An important step to idea that the knowledge of the path taken was as good as a direct observation of that path being taken is enough to prevent interferance due to the "observer effect". You would think that such a clever experiment would warrant a text book or paper somewhere giving credit to someone for it.
We know when and how Faraday did the first experiments on magnetism, why is this piece of history so hard to find – at least I’ve been unable to find it.

ZapperZ

Try:

1.. X.Y. Zou, et al. Phys. Rev. Lett. v.67, p.318 (1991).
2. E. Buks, et al., Nature v.391, p.871 (1998).

There are plenty more of these "which-way" interferometers experiment.

Zz.

RandallB

Nope; these are not what the OP was looking for.
Your just skimming the thread without reading for content.

1.. X.Y. Zou, et al. Phys. Rev. Lett. v.67, p.318 (1991).

"Induced coherence and indistinguishability in optical interference"
X. Y. Zou, L. J. Wang, and L. Mandel

“Second-order interference is observed in the superposition of signal photons from two coherently pumped parametric down-converters, ……….”


2. E. Buks, et al., Nature v.391, p.871 (1998).
"Dephasing in electron interference by a 'which-path' detector"
BUKS E.; SCHUSTER R. ; HEIBLUM M. ; MAHALU D. ; UMANSKY V. (1) ;
“ …..Such a manifestation of the complementarity principle was demonstrated recently using a pair of correlated photons, with measurement of one photon being used to determine the path taken by the other and so prevent …….”

These are “Two Correlated Photons” experiments that are all the rage for some time now.

The request was for Single photon at a time with direct observation at one of the slits (and therefore no observation at the other slit) type experiment, that dividing hits on the screen into two groups indicating which way for each group.

I’ve only heard this represented as being done with electrons not photons, but I’ve never seen a reference to who actually did it, either first or as a confirmation.

ZapperZ

No, they ARE what the OP is looking for.

The problem here is in how one actually determine when a photon passes through one of the slit, or maintain the superposition, without destroying the photon. You can't simply put a photodetector at one of the slit and expect that photon to pass through unimpeded. That is why they used a correlated photon as a "twin". By acting on the twin, and by knowing how they are correlated, one has essentially made a determination when it is passing through a slit, or both slit.

So this is exactly the double slit experiment as requested. Read the paper, not just the abstract.

Zz.

RandallB

Not if you actually read the OP;
“where the of observing the photon or electron going through the slits causes a collapse of the wave function”

Not figuring it out by using one of a pair of photons or electrons as a proxy for detecting the other going through a certain slit. Of course that is nearly impossible to do, that’s why posts 11, 13, 15 referred to the electron version of the experiment as the one claimed by some texts as the one actually being preformed. I recall reading them but don’t recall the books or if they gave credit to anyone doing the actual test.

I’ll draw the picture for you again – I would have though you knew this kind of stuff.
The main point of the experimental proof was not the directly observed electrons but the indirectly observed electrons. As already described you can detect single electrons passing close to just one slit that they have a chance of going through; without detecting electrons close enough to the other slit that they might go through. Using those detections they could match them with individual hits on the pattern detection screen. By detecting half of the screen hits as coming from the tested slit they could know without directly testing of interfering with the other half that they had to have gone through the other slit. Now they had three definable groups of Electron hits to examine for the pattern produced by each the full group and two half groups one path detected the other not directly path detected but still of know slit origin.
The results are expected by us now, but not by them at the time.
When the one slit detection equipment was turned on the full group pattern changed from interference to just dispersion
And of course the detected group pattern also showed no interference, after all those electrons were getting randomly beat up by a photon beam designed to detect them as they entered or exited the slit on one side.
Obviously, the pattern being created by the other half of the electrons was being masked the detected half in the whole larger group – right.
But, no when they examined the pattern of the remaining group of untested at the slit electrons that group also showed just a dispersion pattern.

The point as least implied in the text books is that these types of experiments were the first to indicate that knowledge even obtained indirectly was enough to disturb the wave like character of individual particles.
AND that dealing with this information about how realty worked was an important part of defining HUP and Copenhagen. (Unless Zz you have some reference that they were doing correlated "twin". experiments prior to 1926)

So the question remains – does someone or some several merit credit for actually providing real experimental results that influenced Bohr & Heisenberg. Or were these just thought experiments that were never actually preformed?

I believe they were done, but have not been able to find a record or resource to confirm it. Well I’m going to trust the author when his abstract says they are not using a single particle (one at a time) test.
If you think their abstract is a misrepresentation, provide an excerpt from the full text. I’m not going to subscribe to a service or pay for a paper I don’t need.